Pulse tube cryocoolers consist of a pulse wave generator, which converts electrical energy to acoustic energy, a coldhead which utilizes the acoustic energy to pump heat from a refrigeration load to a warmer heat sink and an inertance network for generating proper phase angle between gas flow and pressure oscillation within the coldhead.
Typically, a non-linear motor is used as the acoustic source and is referred to as a pulse wave generator. The pulse wave generator, coldhead and the inertance network are charged with a gas such as helium. The coldhead has cold and hot heat exchangers to refrigerate a load and to dissipate heat, respectively. The inertance network is typically in the form of a restriction, a compliance volume and an inertance tube connected to the coldhead opposite to the pulse wave generator. The aftercooler is one of the warm heat exchangers in the coldhead and it is used to remove the heat of compression produced by the acoustic source and energy dissipated in the regenerator. The regenerator is a component of the coldhead located between the cold heat exchanger and the aftercooler to absorb the heat from the gas in the compression part of the cycle and to return heat to the gas on the expansion part of the cycle while the gas is reciprocating through the regenerator due to the acoustic wave. The net effect of this process is that heat can be pumped by the gas in the regenerator from a lower temperature area to a higher temperature area.
The operation of the coldhead relies on the proper phasing between the oscillating pressure and the mass flow in the regenerator and thermal buffer tube to pump heat from the lower temperature to the higher temperature. The coldhead and the inertance network have a complex impedance that allows the pulse wave generator to be operated near electromechanical resonance. The conditions at which the coldhead is being run, for instance, refrigeration load, input power, charge pressure affect the complex impedance of the coldhead and inertance network combination and thus the matching of the coldhead with the pulse wave generator. If the pulse wave generator to coldhead and inertance network matching is poor, the pulse wave generator's electric to acoustic energy conversion efficiency will be diminished and the acoustic power that the pulse wave generator is able to generate is therefore reduced. Less acoustic power delivered to the coldhead typically translates into less heat being pumped by the cryocooler and lower cooling capacity.
In the prior art, it is known to control the fine tuning of the inertance network of a pulse tube cryocooler by effectively adjusting the phase between the oscillating pressure and flow in the coldhead. This allows the cryocooler to optimally function and thereby deliver a maximum amount of cooling power to a refrigeration load as is possible. In U.S. Pat. No. 6,666,033, this is achieved by either heating or cooling the flow in a flow restrictor of the inertance network. The heating and cooling of this component changes the temperature, and thus the viscosity and the density of the working fluid in the pulse tube cryocooler. Changing the temperature of the working fluid in the inertance network causes a change in complex impedance of the inertance network components and thus the phase between the oscillating pressure and the flow in the coldhead. In a particular embodiment, an external jacket is provided around the inertance tube and flow restrictor. Control is achieved by the use of adjustable valves to modulate the flow. Heating is achieved by the use of electrical heaters in the cooling jacket. The heating or cooling is controlled in response to the axial temperature profile of the pulse tube by a sensor and a controller.
U.S. Pat. No. 6,021,643 discloses the use of an inertance tube in series with a compliance vessel for an inertance network. A trombone-like sliding tube system can be used to change the dimensions of the inertance tube and thereby provide for a variable complex impedance for tuning the pulse tube cryocooler.
U.S. Patent Application 2006/0086098 describes a method to dynamically adjust the phasing in a regenerative cryocooler such as a pulse tube cryocooler. The cryocooler has a pulse tube, a regenerator, a compressor, and an inertance network. In this patent, the means for adjusting the phasing is through the use of a variable flow restrictor in the inertance network that is constructed using micro electromechanical systems. These flow restrictors may be adjusted dynamically during the operation of a pulse tube cryocooler to allow for optimum cooling during both fast cool down or for steady state operation.
Pulse tube cryocoolers are typically designed for a single narrowly defined operating condition, for example, to cool a refrigeration load to a specific temperature with a specific cooling power equivalent to the heat load. The prior art, discussed above, allows for the dynamic, if not automated control, of the impedance network for the purpose of optimizing the operation of the pulse tube cryocooler to obtain a maximum amount of cooling power from the pulse tube cryocooler under various operational conditions.
The refrigeration load, in practice, can either increase or decrease. This will result in either an increase or decrease in the temperature of the refrigeration load if no adjustment is made to cause a change in the cooling power being delivered by the cryocooler to accommodate the change in the refrigeration load. In either situation, there exists the need to control the pulse tube cryocooler to maintain a set point temperature for the refrigeration load to prevent temperature excursions from the set point. It is also very conceivable for some applications that the set point may be changed or that the cryocooler may be operated in transient conditions where the cryocooler is being used to lower the temperature of the refrigeration load as opposed to just holding a set point. The input power to the acoustic source can be adjusted to change the cooling power being delivered by the cryocooler. Any adjustment of the input power to the cryocooler and refrigeration load temperature away from the design input power and refrigeration load temperature will result in an inefficiency that can become more apparent in large installations.
As will be discussed, the present invention provides a method of controlling a pulse tube cryocooler to maintain the refrigeration load at a set point temperature or to move a refrigeration load to a set point temperature and that ensures a rapid response to increases in the temperature of the refrigeration load.